The present invention relates to a method for manufacturing a display panel in which an amorphous or poly-crystalline semiconductor film formed on an insulating substrate is irradiated with a laser beam so that the film quality is improved or crystal grains are enlarged or transformed to be single-crystalline, and a display panel manufactured in the same manufacturing method, and particularly relates to a method for manufacturing a display panel having active devices such as thin film transistors formed out of a semiconductor film transformed to be single-crystalline, and a display panel manufactured in the same manufacturing method.
In current liquid crystal or organic electroluminescence display units, an image is formed by switching thin film transistors formed out of an amorphous silicon film in a pixel portion (display region) on an insulating substrate of glass, fused quartz or the like. Exponential reduction in manufacturing cost and improvement of reliability can be expected if driver circuits for driving the thin film transistors in the pixel portion, that is, a gate driver circuit portion, a drain driver circuit portion, and a set of other required circuits can be formed concurrently on the insulating substrate.
In the present circumstances, however, the silicon film forming the active layer of transistors is so poor in crystallinity that the performance of the thin film transistors represented by mobility is low. It is therefore difficult to manufacture circuits required to have a high speed and a high function. In order to manufacture such high-speed and high-function circuits, high-mobility thin film transistors are required. In order to realize the high-mobility thin film transistors, it is necessary to improve the crystallinity of the silicon film.
As a method for improving the crystallinity, excimer laser annealing has been used in the background art. According to this method, an amorphous silicon film (whose mobility is not higher than 1 cm2/Vs) formed on an insulating substrate of glass or the like is irradiated and annealed with an excimer laser so that the amorphous silicon film is transformed (reformed) into a poly-crystalline silicon film. Thus, the mobility is improved.
In the poly-crystalline silicon film obtained by irradiation with the excimer laser, the grain size is about several hundreds of nanometers and the mobility is about 100 cm2/Vs. The poly-crystalline silicon film has enough performance to drive the thin film transistors in the pixel portion, but it is deficient in performance to be applied to a driver circuit for driving a display panel, or the like. In addition, projections of several tens to several hundreds of nanometers formed in the grain boundaries depress the voltage durability of the thin film transistors. Further, a wide variation in energy among pulses in the excimer laser narrows the process margin. In addition thereto, due to use of harmful gas, the cost of equipment increases. Moreover, an expensive oscillating tube has to be exchanged periodically so that the running cost increases extremely.
As a solution to the aforementioned problems, Non-Patent Document 1 discloses an annealing technique using a continuous-wave laser.
[Non-Patent Document 1] F. Takeuchi et al. “Performance of poly-Si TFTs fabricated by a Stable Scanning CW Laser Crystallization”, AM-LCD '01 (TFT4-3).